Force and pressure sensors can be produced in a particularly adaptable manner in terms of form and inexpensively from piezoresistive, electrical layers using one of the known layer technologies, for example by fusion using thick-film technology, by vapour deposition using thin-film technology, by bonding on, etc. In a series of known-arrangements, such as e.g. in EP 0 190 270 B1 and in DE 38 18 190 A1, provision is made for allowing the forces or pressures to be measured to act directly on the piezoresistive layer, in which case the deformation body which is required for the likewise known force sensors with strain measuring elements is advantageously omitted. However, in the area of industrial force measurement with a mechanically hard force-introduction part predominantly manufactured from steel, none of the arrangements of this type has proved successful heretofore against the competing devices with deformation bodies or with piezoelectric plates, either because the arrangements do not contain a corresponding force-introduction part at all, such as e.g. in DE 43 00 995 A1 and others, or the force-introduction part specified is not envisaged or is not suitable for the said area of application. The reasons for this are found in the special problems which arise when a force-introduction part composed of comparatively hard and tough material such as steel presses on layers which are brittle or tend to cracking under mechanical stress. As revealed by K. Girkmann, Flxc3xa4chentragwerke [Plane load-bearing structures], Springer-Verlag (1963) p. 72 and others, the forces introduced in the layer under the edges of force-introduction parts lead to jumps and/or spikes in the normal and tangential stress distribution which are steeper and/or higher, the more sharp-edged the force-introduction part is and the more the transverse expansion at the force-introduction area is impeded by friction between the force-introduction part and the substrate. In the substrate, compressive stresses occur under the force-introduction part and tensile stresses occur next to the force-introduction part. In order to avoid undesirable stresses, the pressure-sensitive elements can be arranged in the force bypass, as e.g. in DE 33 33 285 A1 and DE 41 42 141 A1, as a result of which, however, the sensitivity of the arrangement is disadvantageously reduced.
The narrow marginal zones in the layers directly under and next to the edges of a force-introduction part, as is described e.g. in DE 39 12 280 A1, at any rate experience considerable mechanical loading which is particularly disadvantageous in the tensile-stress region right next to the edges of the force-introductionpart, because the tensile strength of thick-film material is more than a power of ten less than its compressive strength. In the event of continual dynamic loading, the cracking in the layer begins in these marginal zones, and has an adverse effect on the long-term stability of the sensor.
DE 38 18 191 A1 and DE 42 21 426 C1 furthermore disclose force sensors in which the force-introduction part also presses on the regions of the layers which overlap the conductor tracks. Since the layer is electrically connected in parallel with the conductor track in this overlap region, the force component introduced there effects a change in resistance which is not detected, in other words a force bypass that is not concomitantly measured. Moreover, these arrangements have the disadvantage that the transverse expansions (perpendicular to the force direction) occurring in the sensor element on account of the three-dimensional elasticity lead to transverse forces on the conductor tracks and, on the other hand, within the resistive tracks there arise instances of transverse expansion impeding due to the delimiting conductor tracks.
DE 42 21 426 C1 furthermore discloses a variant of a pressure sensor in which the layer covers the conductor tracks and, in addition, a bearing region which is situated between these and is higher than the conductor tracks. The effect achieved by the force-introduction part projecting above the bearing region is that the edges thereof cannot have disturbing or damaging effects on the layer. However, this arrangement has the disadvantage that it is necessary to produce a separate bearing region whose height exceeds that of the conductor tracks to such a distinct extent that in the event of any conceivable loading, the force-introductionpart does not end up placed on the conductor tracks. As a result of this, the force-introductionpart turns out to be at an increased distance from the support body, which impairs the stability properties of the arrangement.
The invention is based on the object, then, of specifying a force sensor containing a support (4), a pressure-sensitive layer (1a), which is arranged above the support (4) and is connected to a conductor track (3a), a force-introduction part (5a) with a contact area which is bounded by edges and is connected to the layer (1a) in the bearing region (2a), in the case of which force sensor force bypasses, instances of transverse expansion impeding and cracking owing to the effect of edges of the force-introduction parts are precluded as far as possible by means of a suitable construction. Furthermore, the intention is to improve the measurement precision of the force sensor. Finally, the intention is to avoid the sheet-metal bulging and transverse effects that occur with single-sided coating.
This object is achieved by virtue of the fact that a second pressure-sensitive layer (1b), which is connected to a conductor track (3b), is arranged on the support (4), and that a second force-introduction part (5b) is connected to the second pressure-sensitive layer (1b) in the bearing region of a contact area bounded by edges.
Accordingly, the force sensors according to the invention contain a support serving for the construction of the entire element, and also two pressure-sensitive layers which are arranged above the support and are connected to conductor tracks. In this case, if appropriate, further material layers can be arranged between the support and the layers. Furthermore, the force sensors contain force-introduction parts with contact areas which are bounded by edges and are connected to the layers in the bearing region. The force-introduction parts can have virtually any desired form, depending on the requirements of the application. Via their outer areas, the contact areas, they are connected (directly or via intermediate layers) to the pressure-sensitive layers of the support, although the contact takes place only via the inner region of the contact area, that is to say not via the edges. This prevents damage to the pressure-sensitive layer of the support by the edges.
Those surfaces of the layers of the support which face the force-introduction parts may have grooved depressions above which the edges of the contact areas lie in a manner free from contact. In contrast to the prior art, then, it is not necessary to provide a bearing region which is higher than the conductor tracks. Rather, it is possible to arrange pressure-sensitive layers on the support, which layers have a uniform (except for grooves), even height.
Single-layer or multilayer elevated bearing regions and, spaced apart from the latter by interspaces, conductor tracks may also be arranged on the support of the force sensor, conductor tracks, interspaces and bearing regions being covered by the layers in such a way that the grooved depressions are formed above the interspaces. In this case, the bearing regions are preferably formed integrally from the material of the support.
The conductor tracks may also be arranged with their whole area between the support and the pressure-sensitive layers. In the case of an arrangement of this type, the electric current conducted into the layers through the conductor tracks flows orthogonally through the layers, that is to say, as a rule, in the direction of the effect of the compressive force. In this case, at least one of the conductor tracks is preferably formed by the electrically conductive material of the support. In order to compensate for surface irregularities, this is preferably done with the interposition of suitable compensation layers.
The sensor according to the invention therefore involves a xe2x80x9csandwichxe2x80x9d design in which two pressure-sensitive layers connected to conductor tracks are arranged on the support, and in which two force-introduction parts are connected to the two pressure-sensitive layers at the inner regions of contact areas bounded by edges. As a rule, the support is designed to be so small that the measuring units arranged on it lie spatially close together.
The mechanical and electrical coupling of two measuring units affords considerable advantages in respect of precision. It is also conceivable to use the xe2x80x9csandwichxe2x80x9d design a number of times, so that not just two but a plurality of sensor subunits are coupled to the same intermediate element.
The pressure-sensitive layers on the support may be composed of a material which supplies an electrical signal as a function of the acting pressure. This may be, by way of example, a piezoelectric or piezoresistive material or a semiconductor material. This material is advantageously one which is used as standard in semiconductor fabrication, such as, for example, a disk-seal diode.
Furthermore, the force-introduction parts and/or the support may be provided with an insulating coating. This is the case particularly when the said elements themselves are composed of an electrically conductive material.
For the coating it is possible to use insulating joining materials such as, for example, adhesives with a time-dependent curing process or glass ceramic sheets, so-called xe2x80x9cceramic tapesxe2x80x9d, in order that the entire sensor arrangement can advantageously be pressed on one another in a plane-parallel manner before the curing or firing.
Likewise insulating joining materials, for example adhesives with a time-dependent curing process or glass ceramic sheets (so-called xe2x80x9cceramic tapesxe2x80x9d), can be used as materials for the support. In this case, too, the entire sensor arrangement can advantageously be pressed on one another in a plane-parallel manner before the curing or firing.